The shoulder joint is formed by the ball-shaped humeral head of the humerus rotatably being received within the glenoid cavity of the scapula. A group of tendons and muscles at the rotator cuff extend between the scapula and the humerus to maintain the humeral head within the glenoid cavity. Disease and overuse can cause the rotator cuff muscles and tendons to become damaged, weakening the connection between the humeral head and the glenoid cavity and permitting the humeral head to shift or track unnaturally within the glenoid cavity. The irregular movement of the humeral head can damage the humeral head or the scapula, requiting repair or replacement of the humeral head or the bone surrounding the glenoid cavity.
In conventional shoulder replacement procedures, a humeral implant having a ball head and/or a glenoid implant defining a socket are mounted to the humerus and scapula, respectively. The conventional humeral and glenoid implants are intended to supplement or replace the existing bone structure damaged by the irregular movement of the humeral head. As the conventional implants are substitutes for the natural structures, conventional shoulder replacement procedures require the rotator cuff muscles and tendons to be sufficiently undamaged or healed to maintain proper contact between the humeral head and glenoid cavity and prevent irregular movement of the humeral head.
A reverse shoulder replacement procedure can be employed if the rotator cuff muscles and tendons are too damaged for a conventional procedure. in reverse shoulder replacement, a glenoid implant having a ball head, or glenosphere, can be mounted to the scapula and a humeral implant defining a socket is mounted to the humerus. The glenoid and humeral implants reverse the ball and socket of the natural shoulder joint such that the deltoid muscle, rather than the damaged rotator cuff muscles and tendons, can be used to maintain the joining of the humerus and scapular.